In recent years, with a performance improvement of industrial apparatuses, the outputs of power modules mounted thereon have been becoming higher. Accordingly, the outputs of semiconductor elements have been becoming higher. Operation guaranteed temperatures of semiconductor elements are, for example, not lower than 125° C. nor higher than 150° C. It is expected that the operation guaranteed temperatures will increase to 175° C. or higher in the future.
The increase in the operation guaranteed temperatures of semiconductor elements requires that circuit boards should have high thermal cycle test (TCT) characteristics. In TCT, low temperature→room temperature→high temperature→room temperature is one cycle and durability of a ceramic circuit board is measured.
There has been disclosed a ceramic circuit board having no void in a protruding portion of a brazing material. It is described that a ceramic circuit board having a silicon nitride substrate has 5000 cycle-durability. By eliminating the void in the protruding portion of the brazing material, it is possible to improve the TCT characteristics. However, with the performance improvement of semiconductor elements, it is expected that the operation guaranteed temperature will increase to 175° C. or higher.
Meanwhile, with the performance improvement of semiconductor elements, studies are made on the use of a large external terminal in order to increase ampacity. It is known that the thickness of a joined portion of a conventional external terminal is set to 3 mm or more.
External terminals include those having various shapes such as a lead frame and a lead pin. To increase the ampacity, using a relatively thick lead frame instead of a thin wire such as a bonding wire is effective. The use of the thick lead frame, however, may cause the ceramic circuit board to greatly curve. If the ceramic circuit board curves, poor mounting is likely to occur when it is mounted on a heat dissipation member. Further, when the ceramic circuit board having lead terminals is subjected to TCT whose temperature in the high temperature state is 175° C., it is difficult to obtain good characteristics. This is because a higher stress is applied to the ceramic circuit board as the lead frame is thicker. Under such circumstances, there is required a ceramic circuit board with good TCT characteristics that does not curve even when a thick lead frame and a terminal are joined thereto.